Summing amplifier



Patented June 11, 1946 UNITED STATES PATENT OFFICE SUMMING AMPLIFIERApplication May 1, 1941, Serial No. 391,331

11 Claims.

This invention relates to electrical calculating devices andparticularly to a device for obtaining the sum of a plurality ofelectrical voltages.

The object of the invention is to obtain the sum of a number ofelectrical voltages, one pole of each of the voltages being grounded.

A feature of the invention is an electrical amplifier having a feedbackof power from the output circuit to the input circuit of the amplifierof such magnitude and phase as to reduce the input impedance of theamplifier to a small value and to make the over-all gain of theamplifier a predetermined quantity.

In prior electrical circuits for the addition of quantities representedby electrical voltages, it has been necessary to place the voltages tobe added in serial relationship in order to obtain the sum of thesevoltages. Such a connection has the serious practical disadvantage thatonly one of the voltages can be connected to ground. In accordance withthe present invention all of the sources of voltages may be connected toa common point, preferably to ground, and supplied through individualhigh impedances to the input of an electrical network.

The voltages to be added may be of any desired frequencies, and may havezero cycles, that is, a direct voltage, as one limit. The network isdesigned to amplify voltages within the desired frequency range, withoutdistortion or instability, and has a feedback from the output circuit tothe input circuit of such polarity and magnitude as to render theimpedance of the input circuit, as viewed from the voltage sources, lowcompared to the impedances in series with the sources, without renderingthe amplifier unstable. In the specific embodiment of the inventiondisclosed in the present application, the network is capable ofamplifying voltages from zero cycles to a comparatively high frequency,but the invention is in no-way limited to this specific disclosure, asthe interstage coupling networks of the amplifier may be designed byknown methods to amplify any other desired range of frequencies. Thenetwork is further adjusted so that the overall gain is a predeterminedquantity, and during the operation of the circuit, the network will tendautomatically to maintain this relationship.

Thus the sum of the voltages will be reproduced across the output load,without any interaction of one source of voltage upon another. Further,by adjustment of the impedances connected in series with the varioussources of voltage, any one or more of the sources may be, in effect,multiplied by any desired factor and this voltage, multiplied by suchfactor, will be included as one element of the summation of voltages inthe output circuit.

The drawing diagrammatically illustrates a circuit embodying theinvention.

In the drawing the generators A, B, and C, diagrammatically symbolizingthree sources of voltages to be added, are respectively connected inserial relationship with one of the impedances l, 2, 3, each having arelatively high impedance compared to the effective input impedance ofthe amplifier. The impedances I, 2, 3 will normally be resistors, unlessthe added voltages are to be modified in accordance with frequency, inwhich case the impedances may have any desired frequency characteristic.

The voltage sources, and their serially connected impedances, areconnected in parallel. relationship to the input of a thermionic vacuumtube 4. This vacuum tube may, if desired, have the usual grid biasingresistor 5. The vacuum tube 4 is coupled to the vacuum tube 6 by meansof an interstage coupling network comprising the three resistors l, 8and 9. This coupling network is of the type disclosed in U. S. Patent1,751,527, March 25, 1930, H. Nyquist, but any pther form of couplingnetwork capable of operation without distortion for voltages coveringthe frequency range desired may be used in place of the network shown.The vacuum tube 6 is coupled to the vacuum tube III by a similar networkcomprising the resistors ll, 12 and [3, respectively. The vacuum tube I0is coupled by means of a resistor I4 to the load I 5. The anode of thevacuum tube III is coupled by an impedance IE to the control electrode,or grid, of the vacuum tube 4, feeding back energy from the outputcircuit of the vacuum tube l0 to the input circuit of the vacuum tube 4.As this amplifier comthe energy fed back will be in the proper phase tooppose the voltage applied to the input of the vacuum tube 4, thusforming a reverse feedback.

An intermediate tap of the battery 25, or other suitable source ofvoltage, is connected to the grounded connection 26. Positive potentialfrom source 25 is supplied through resistors 1, II, to the anodes ofvacuum tubes 4, 6, and negative potential from the source 25 is suppliedthrough resistors 9, l3 to the control electrodes of vacuum tubes 6,Ill. The screen grid of vacuum tube 6 is connected to a suitable tap inthe source 25. Positive potential is supplied through coupling resistorI4 to the anode of vacuum tube I0, and negative potential from thesource 25 is supplied to the cathode of vacuum tube Hi. The screen gridof vacuum tube In is grounded, thus making the screen positive withrespect to the cathode of vacuum tube Ill.

The operation of vacuum tube In may be explained in different ways, someof which are set forth hereinafter but the scope of the invention is notthereby limited to these theories of operation. In the absence of anapplied signal, the 60 constants of the circuit may be so adjusted thatprises an odd number of stages of amplification 3 the positive potentialfrom the source 25 is completely used up in driving the anode currentthrough resistor It so that no voltage is applied to the anode of vacuumtube l0, which is thus at ground potential. The anode current of vacuumtube I is maintained by the negative potential from the source 25applied to the cathode of vacuum tube [0. When a negative voltage isapplied to the control electrode'of vacuum tube t, the amplifiedvoltagewill cause the control electrode of vacuum tube ID to become morenegative, reducing the anode current and the voltage drop in resistorIA, and applying a positive voltage to the load Hi. When a positivevoltage is applied to the control electrode of vacuum tube Q, theamplified voltage will cause the control electrode of vacuum tube ID tobecome less negative, permitting the negative voltage applied to thecathode of vacuum tube In to increase the anode current, making thevoltage drop in resistor It largerthan the applied positive potentialfrom the source 25, and applying a negative voltage to the load.

From another point of view, a current can flow from the positive tap ofthe source 25 through resistor M, down through load 15, and connection26 back'to the source 25. Another current can a flow from the tap ofsource 25, by connection 26 up through load i5, anode to cathode ofvacuum tube In to the negative tap of source 25. By adjusting the biason the control electrode of vacuum tube It, these curlents, in theabsence of a signal, may be made e nal. Thus no current will flow in theload I5, and the anode of vacuum tube in will be at ground potential.When an amplifled signal is applied to the control electrode of vacuumtube H), the balance of these'theoretical currents is disturbed, and aresultant current will flow in the load l5.

Let quantities relating to the sources A, B, C be designated bysubscripts a, b, c, and those relating to the output circuit bysubscript d,

The control electrode, or grid, of vacuum tube t has a potential,preferably negative, suchthat the control electrode does not draw anyappreciable current.

Applying the Kirchofi' relationship to the node at the control electrodeof vacuum tube Q,

Let the impedances 1, 2, 3, 16 be resistances R1, R2, R3, Rm, thepotential of the control electrode of vacuum tube 4 be 8g and thevoltage across the output circuit be ed, then Solving Equation 3 for is,is, i0 and id and substituting in Equation 2,

where p. is the voltage amplification ratio of the amplifier.

If h be large compared to unity, the bracketed are unity, and that theratio a is 6x10 the denominator of Equation 4 will be which differs fromunity by about .007 per cent. The coupling impedance formed by the inputcircuit of vacuum tube 4 may be shown to be 1+1 If is 6x10 and rm is 1megohm, the coupling impedance is about 15 ohms. If the input resistorsare of the order of 1 megohm each, the interaction between the sourcesA, B, C is negligible. The output impedance of the vacuum tube 10 isreduced by the factor ia 10 "'10 1 z a) and is effectively less than 10ohms. The amplifier, in effect, forms a voltage source of very lowimpedance, thus variations in the load impedance have little effect onthe accuracy of the summation.

As the effective gain for each source is controlled by the ratio of thefeedback resistorto the input resistor, the voltages from the sourcesmay have different gains, thus multiplying or dividing one voltage withrespect to the others.

For the purpose of illustrating the flexibility and utility of theinvention and not as any limitation thereon, let the impedances 1, 2, 3be resistances R1, R2, R3, and let the impedance 16 be a resistance R4,and let D be the voltage across the load [5.

If R =R -xR =R then D=A+B+:0C where m is any desired factor. Thus, if Rbe the feedback impedance, and R4 be the input impedance for, thevoltage E1, the voltage gain G for that input will be and thisrelationship will apply simultaneously and independently for all inputs,thus for any When the network is designed for use with voltages havingfrequencies which may go down to' zero cycles, preferably the circuitconstants are so chosen that in the absence of an input voltage thepotential of the anode of the vacuum tube In is substantially that ofthe ground, or zero voltage. When voltages are applied to the inputcircuit of the vacuum tube 4 the potential of the anode of the vacuumtube ID will swing above or below the ground potential in accordancewith the sum of the voltages applied to the input of the vacuum tube 4,consideration being taken of the signs of the voltages applied.

In order to adjust the potential of the anode of the vacuum tube l0exactly to zero or ground potential, a correcting circuit comprising anyconvenient source of voltage E supplied to the potentiometer i1 may beconnected through a serial resistance 18 to the input of the vacuum tube4, and, in the absence of other applied voltages, the potentiometer maybe adjusted to bring the potential of the anode of the vacuum tube Inexactly to the zero or ground potential. As the eflective gain of thecomplete network depends upon the ratio of the resistance of theresistor in series with any source of voltage compared to the resistanceof the feedback resistor IS, in order to secure a more accurate controlof the adjustment, the resistance of the resistor I8 is preferably madesay three or four times the resistance of the resistor I 5. Thisrelationship will also improve the direct current noise situation.

In a practical embodiment of the invention the resistors I, 2 and 3 were1 megohm. The potentiometer I! was of the order of 10,000 ohms and theresistor l8 was 3 megohms. The vacuum tubes 4, 5 and were commercialvacuum tubes having the type designations respectively BSC'I, 6SJ7, and6Y6G. The resistor 5 was 1,500 ohms, the resistors 1, 8 and 9 wererespectively and 2 megohms. The resistors ll, l2 and It wererespectively V 1 and 1 megohms. The resistor 14 was 6,000 ohms and theload l5 was also 6,000 ohms. The resistor It was 1 megohm. A ZOO-voltbattery grounded at the mid-point was used, plus 350 volts beingsupplied to the anodes of the vacuum tubes 4 and 6, plus 250 volts tothe anode of the vacuum tube l0, plus '15 volts to the screen of thevacuum tube 6, ground or zero volts to the cathode of the vacuum tube 8and the screen rid of the vacuum tube III, minus 135 volts to thecathode of the vacuum tube It, and minus 350 volts to the grid biasingresistors of the vacuum tubes 6 and I0.

To improve the stability of the amplifier, and to obviate high frequencysinging due to parasitic capacitances, small capacitors i9, 20 may beconnected from the anodes of the vacuum tubes 4 and 0 to the cathodecircuit, and small capacitors 2 I, 23, respectively, in serialrelationship with resistors 22, 24, may be connected across the inputcircuits of the vacuum tubes 0 and I0. The capacitors l0, 2|, 2|, 23were respectively .25, .0001, .001, .03 microfarad and the resistors 22,24 were 100,000 ohms.

What is claimed is:

1. In combination, a plurality of voltage sources, a plurality of highimpedances respectively in serial relationship with said sources, anamplifying device having an input and an output circuit, said sourcesand impedances being connected in parallel relationship to said inputcircuit, a load impedance in said output circuit, and means for feedingback energy from said output circuit to said input circuit to make theimpedance of said input circuit small compared to said input impedancesand the over-all gain of said amplifier substantially unity.

2. In combination, a plurality of voltage sources, a plurality of highresistances respectively in serial relationship with said sources, anamplii'ying device having an input and an output circuit, said sourcesand resistances being connected in parallel relationship to said inputcircuit, a load resistor in said output circuit, and means including ahigh resistance for feeding back energy from said output circuit to saidinput circuit to make the over-all gain of said amplifier for any one ofsaid sources substantially equal to the ratio between the resistancefeeding back energy and the resistance connected in serial relationshipwith that one of said sources.

3. In combination, a thermionic device having a cathode, an anode and acontrol electrode, a coupling impedance, a source of current connectedto said cathode and through said coupling impedance to said anode andhaving an intermediate tap, an output circuit connected to said anodeand said intermediate tap, two impedances connected in serialrelationship across said output circuit, and a connection from thejunction of said two impedances to said control electrode.

4. The combination in claim 3 with amplifying means in said connectionfrom the junction of said two impedances to said control electrode.

5. The combination in claim 3 with a source of control voltage in serialrelationship with said two impedances.

6. In combination, an electron discharge device having at least acathode and an anode, a source of direct current having the negativepole connected to said cathode and the positive pole to said anode, anoutput circuit connected to said anode and an intermediate point in saidsource, two impedances connected in serial relationship across saidoutput circuit, said intermediate point being so chosen that thecurrents from said source flowing in said impedances are substantiallyequal and opposite and the voltage across said output circuit issubstantially zero.

7. The combination in claim 6 with a connection from the Junction ofsaid serially connected impedances to a control electrode in saidelectron discharge device.

8. The combination in claim 6 with a source of voltage in serialrelationship with said serially connected impedances adjusted to makethe voltage across said output circuit more nearly equal to zero.

9. In combination, an electron discharge device having at least acathode and an anode, a source or direct current having the negativepole connected to said cathode and the positive pole connected to saidanode, an output circuit connected to said anode and an intermediatepoint in said source, a plurality of voltage sources. a plurality ofimpedances respectively in serial relationship with said voltagesources, said impedances and voltage sources being connected in parallelrelationship, and another impedance connected in serial relationshipwith said parallel connected impedances and voltage sources across saidoutput circuit.

10. The combination in claim 9 with a connection from the junction ofsaid serially connected impedances to a control electrode in saidelectron discharge device.

11. A network having a plurality of input and output terminals, apotential divider connected in serial relationship with a plurality ofsaid input terminals and one of said output terminals, said dividerhaving a plurality of impedances in parallel relationship connected inserial relationship with a single impedance, an electron dischargedevice having at least a cathode, an anode and a control electrode, adirect connection from the Junction of said plurality of impedances andsaid single impedance to said control electrode, a connection from saidanode to said one output terminal, and a connection from said cathode toan input terminal and the remaining output terminal.

KARL D. SWARTZEL, JR.

